Stabilized transferred electron amplifier

ABSTRACT

A supercritically doped pulsed transferred electron device is described in which means are provided for thermally inducing stability in addition to circuit loading to achieve wideband linear amplification.

United States Patent [151 3,686,578 Upadhyayula et al. [451 Aug. 22, 1972 [54] STABILIZED TRANSFERRED [56] References Cited ELECTRON AMPLIFIER UNITED STATES PATENTS [72] Inventors: Chainulu Lakshminarashimha Upadhyayuh, Cranbm-y; Ban-y 3,644,839 2/1972 Perlman et al ..330/5 Stuart Perlman, l-lightstown, both of Assignee: RCA Corporation Filed: Jan. 25, 1971 Appl. No.: 109,159

Field of Search ..330/5 Primary ExaminerRoy Lake Assistant Examiner-Darwin R. Hostetter Attorney-Edward J. Norton [57] ABSTRACT A supercritically doped pulsed transferred electron device is described in which means are provided for thermally inducing stability in addition to circuit loading to achieve wideband linear amplification.

2 Clains, 4 Drawing Figures Patented Aug. 22, 172

2 Sheets-Sheet 1 w k \x Q km @5 .RRRQ g W Qf STABILIZED TRANSFERRED ELECTRON AMPLIFIER The invention herein was made in the course of or under contract or subcontract thereunder with the Department of the Army.

This invention relates to transferred electron devices and more particularly to bulk-type transferred electron devices wherein the active region is supercritically doped.

Supercritically doped bulk-type transferred electron oscillator devices are known and are presently considered for use as high power solid state microwave signal generators. The term supercritically doped refers to transferred electron devices such as those made of epitaxial Gallium Arsenide (G,,A,) where the bulk region has an nL product at room temperature greater than about 5 X l cm- The term n in the above product is the carrier density of the bulk material and L is the length of the sample. When such a bulk material has a do. (direct current) electric field thereacross that exceeds a given threshold, suchv as about 3 kilovolts per cm (centimeter), for example, the drift velocity of the electrons as a function of the electric field decreases. The transfer of electrons from high velocity states to low velocity states takes'place in a relatively short time compared to the frequency of the microwave signals, giving rise to a bulk negative resistance. If the bulk material is supercritically doped, dipole layers form at or near the cathode of the material and move through the material with the drift velocity of the electron stream and disappear at the anode whereupon a new domain is formed and the process is repeated. The fundamental frequency of oscillation will be approximately equal-to the transit time frequency The transit time frequency f, equals V IL, where V is the average drift velocity of the electron stream and L is the length of the active material between the terminals. While power oscillators have been associated with supercritically doped material biased above threshold, wideband amplification of signals at or near the transit time frequency using such materials was previously unknown before that described by Barry Perlman and Thomas Walsh. This amplifier is the subject of a copending US. Pat. application, Ser. No. 888,477, filed on Dec. 29, 1969, now U.S. Pat. No. 3,644,839, by Perlman and Walsh. Prior to this time, amplification of signals at or near the transit time frequency using such materials was generally considered not possible due to what was considered an inherent steady oscillating state associated with dipole instability.

It is difficult and extremely costly to produce in practice at this time uncompensated material where the doping density is less than about 5 X l0 cm' on a production type scale. For practical purposes therefore the lowest doping density is that of about 5 X When the device is operated at an X-band transit time frequency, the length L of the diode is on the order of 10 microns. When the diode is operated at C-band, the diode length L is on the order of 20 microns. This results in an id. product of 5 X 10 to 1 X l0 cm As stated previously, these 111. products before Perlman and Walsh were associated with instability.

In the arrangement described by Perlman and Walsh in application, Ser. No. 888,477 filed Dec. 29, 1969, a CW amplifier operating at room temperature is described. This was the first such amplifier which produced amplification, where the nL product of the device at room temperature was greater than 5 X l0 cmand where the amplification was observed at or near the transit time frequency. In this amplifier, the biasing voltage was a steady dc. voltage at a level of about two to three times threshold. The amplifier arrangement in Ser. No. 888,477 also discloses generally the use of this amplifier for amplifying pulse signals as well as CW signals. Success of the arrangement led to further studies with regard to this type of transferred electron amplifier. At a later time, when attempting to build a relatively low duty cycle 1 percent) pulsed amplifier at room temperature, the amplifier ceased to provide linear amplification of applied RF microwave signals even when the amplifier was pulsed with l microsecond electric field bias pulses of 40 volts. This bias voltage approaches breakdown of the diode.

Briefly, the present invention provides a stabilized transferred electron pulsed amplifier for amplifying microwave signals at or near the transit time frequency. The amplifier includes a bulk-type device having first and second terminals on opposite ends of the device. The device has an active region between the terminals wherein at room temperature the product of the doping density times the length of the active region between the terminals is greater than 5 X l0 cm An electric field is established across the device that is beyond the threshold where the transfer of electrons from high mobility sub-bands to low mobility sub-bands occurs. This device is properly loaded and the temperature is controlled to stabilize the device and to provide wideband linear amplification of the microwave signals.

DETAILED, DESCRIPTION A description of an embodiment of the present invention follows in conjunction with the following drawings wherein:

FIG. 1 is a sketch, partly in cross section, of a transferred electron amplifier,

FIG. 2 is a cross section of the diode package,

FIG. 3 is a measured carrier density profile of a portion of the acfive region of a typical device, and

Fig. 4 is a block diagram of a test facility with the amplifier of FIG. 1.

Referring to Flg. 1, there is illustrated a reflectiontype amplifier 10 in accordance with one embodiment of the resent invention in a temperature controllable oven 40. A bulk Gallium Arsenide (6 A,) negative conductivity device package 11 is mounted in a coaxial transmission line structure 13. The coaxial transmission line structure 13 includes a center conductor 15 spaced from an outer conductor 17 by a low dielectric constant disk tuner 14. The dielectric constant of the disk tuner 14 is about 2.1. The disk tuner 14 is about onequarter wavelength long at the center operating frequency of the linear amplifier l0 (6 6H,, for example). The center conductor 15 and outer conductor 17 are dimensioned and arranged to provide about a 30 to 35 ohm transmission line and to provide a transition structure from 50 ohms at the input to about 17 to 20 ohms at the device package 11. In the example illustrated in Hg. 1, the center conductor 15 is mils in diameter and the inner diameter of the outer conductor 17 is 230 mils. The outer conductor 17 is coupled to ground potential.

Terminating one end of the transmission line 13 is a reflective conductive termination 19. The conductive termination 19 also serves as a heat sink. The device package 11 is mounted such that the device package 11 is connected between the center conductor 15 and the conductive termination 19 with the one terminal 18 coupled to conductive termination 19 and the opposite terminal 20 connected to the center conductor 15. Proper electric field bias across the device package 11 is provided by a controlled pulse source (not shown) applied to terminal 21. The bias pulse from the source is coupled through feedthrough capacitor 23 and RF choke coil 25 to the center conductor 15. A piece of dielectric material 27 separates the sections 150 and 15b of the center conductor 15 to form a capacitor 29. Capacitor 20 acts both as an RF coupling capacitor for the input and reflected RF microwave signals and to block the bias pulses from the output. The bias source is isolated from the RF energy source by means of the RF choke coil 25 and the RF bypass capacitance provided by the feedthrough capacitor 23. Impedance transformation from 50 ohms at input to about 17 to 20 ohms at the device 11 is achieved by the 30 to ohm line made up of inner conductor 15 and outer conductor 17 and the disk tuner 14. The disk tuner 14 is used to provide only slight impedance changes and to sufficiently load the amplifier to suppress oscillation. The disk tuner 14 has a length along the direction of propagation of signals therealong equal to about onequarter of an electrical wavelength at an operating frequency of the amplifier (6 GH,, for example) so as to provide a distributed quarter-wave impedance transformer.

A capacitive probe tuning screw 71 extends through outer conductor 17 and is spaced a selected distance from inner conductor 15. This tuning screw 71 allows slight amplifier tuning to achieve a uniform gain over the desired operating band of frequencies.

RF microwave input signals are coupled into and out of the reflection-type amplifier 10 by means of a circulator 35 in FIG. 1. The circulator 35 may be, for example, a strip transmission line circulator of the type having three center conductors joined together at a common conductive region and a pair of flat outer conductors spaced from the center conductors. A body of ferrite material is positioned between each of the outer conductors and the common conductive junction region of the center conductors. A pair of magnets are positioned above and below the ferrites to provide sufficient d.c. magnetic bias. One such circulator is described by Davis in US. Pat. No. 3,063,024.

The center conductors 38, 42 and 46 of coaxial lines 37, 39 and 41 are connected each to one of the three joined center conductors of the circulator 35. The outer conductors of circulator 35 are coupled to ground potential. The outer conductors 36, 43 and 44 of lines 37, 39 and 41 are connected to the grounded outer conductors of circulator 35. The center conductor 38 of coaxial line 37 is connected to section 15b of center conductor 15. The outer conductor 36 is connected to outer conductor 17 by means of conductive plate 16. The center conductor 15 is tapered to meet with center conductor 38. The ratio of the sizes of the inner and outer conductors of the coaxial lines 37, 39 and 41 is arranged so each is a 50 ohm line.

In the operation of the circulator arrangement of FIG. 1, RF microwave input signals are applied along line 39 to one arm of circulator 35 and are coupled non-reciprocally by the action of the circulator along line 37 toward amplifier 10. The reflected amplified RF microwave signals propagate along transmission line 13 and along line 37 to circulator 35. These signals at the circulator 35 are coupled nonreciprocally out of the circulator along line 41.

Referring to FIG. 2, there is shown in cross section the device package 1 1. The device package 1 l is about mils long and about 50 mils in diameter. The device package 11 comprises a hollow cylindrical body 45 of insulator material. A disk 47 of conductive material covers one end of the body 45. This disk 47 of conductive material is at terminal 20 in FIG. 1 and is positioned adjacent to the center conductor 15. A conductive heat sink 19 having a thin layer 49 of gold thereon is positioned across the terminal 18 end of the body 45 of insulator material. An n-n-n epitaxial Gallium Arsenide sandwich structural device 51 is mounted within the body 45. One n layer 53 has a layer 55 of silver thereon to provide a first terminal of the device 51. The device 51 is mounted such that the layer 55 of silver is placed adjacent to and in contact with the layer 49 of gold. The other 'n layer 57 has a layer 59 of silver thereon to provide the second terminal of the device 51. Gold wires 62 and 63 are connected between the silver layer 59 and the conductive disk 47 which is adjacent to center conductor 15. Since both the bias pulses and the RF microwave signals are applied along the center conductor 15 connected to conductive disk 47 and wires 62 and 63, both the bias pulses and the RF microwave signals are applied along the wires 62 and 63 to the device 51 at terminal layer 59.

The device 51 was constructed as illustrated of a nn-n configuration. The total spacing between the silvered ends 55 and 59 is about 26 microns long. The n layer 65 is the active layer and this is 20 microns long. The n layer 53 is about 4 microns and the n layer 57 about 2 microns. The dimension of the device 51 as viewed at the silvered ends is 17 X 17 mils.

The device was epitaxial Gallium Arsenide (G,,A,). The n -n-n geometry was grown using a vapor hydride process. The details of the growth process using arsine has been described in the following articles:

J. J. Tietjen and J. A. Amick, Preparation and Properties of Vapor Deposited Epitaxial GaAs (l-X) Using Arsine and Phosphine,

J. Electrochem. Soc., 113, p. 724, (1966), and

R. E. Enstrom and C. C. Peterson, Vapor Phase Growth and Properties of GaAs Gunn Devices, Trans. Metalurgical Soc. AIME American Institute of Metalurgical Engineers), 239 p. 413, (1967.).

The n layer for C-band of frequency as mentioned above was 20 microns and the doping densities at room temperature were about l X l0 cm. The material was not compensated and, consequently, is characterized by a positive temperature coefiicient. Room temperature Hall measurements were made on the device just before and just after the device was grown. Room temperature mobilities were typically 6,000 cm per volt second. Doping profiles at room temperature on the device were made. FIG. 3 shows a partial measured doping density profile for a typical 16 micron n layer 65. The profile is of a 16 micron n layer from a point about 3 microns within the n layer from the n layer 53 end to the n* layer 57. The rapid rise in carrier density is at the interface with the n' layer 57. For the 20micron n layer device, the rapid rise in carrier density would occur at about 20 microns from n layer 53.

Referring to FIG. 4, there is illustrated a block diagram of the test facility for the amplifier in a temperature controllable oven 40. A sweep frequency generator 61 is connected to directional coupler 63 which, in turn, is connected by line 39 to circulator 35.The circulator 35 is connected to the temperature controlled transferred electron amplifier 10 through line 37. The transferred electron amplifier 10 is biased by an electric field voltage pulse from bias source 77.

The reflected output from the transferred electron amplifier 10 is coupled along line 37 to circulator 35 and directed by circulator 35 as an output along line 41. Coaxial line 41 is connected to directional coupler 65. The directional coupler 65 is connected at one output temiinal to oscilloscope 67 and at the other output terminal through bolometer 69 to a power output meter 71. Metering from the output of the frequency generator 61 is provided by the coupling of the power input meter 75 and the bolometer 73 to one terminal of directional coupler 63.

Referring to FIGS. 1 and 4, the amplifier 10 was operated and tested by first attempting to operate the amplifier 10 at room temperature by having the ambient temperature of the oven at room temperature. The ambient temperature of the device was then raised by raising the temperature in the oven 40 and monitoring it by a thermocouple coupled to the heat sink 19.

When the oven was at room temperature, a l microsecond bias pulse from the bias source 77 was provided at a 10 KC rate. The amplitude of this bias pulse was to a level of threshold which, for the micron n layer device described above, was about 6 volts. At threshold, without any RF signaL output from the frequency generator 61, oscillations at the output were noted on-the oscilloscope 67 and were measured at power output meter 71.

The amplitude of the bias pulse from source 77 was further increased beyond that of two to three times threshold. Tuning disk 14, shown in Fig. l, was moved toward and away from the device package 11 until a condition was reached where the oscillations ceased, as noted by no output measured at the power meter 71.

Gated 4 to 8 GH, (Gigahertz) signals were provided at the output of the sweep generator 61. The 4 to 8 GH, frequency signals were gated out of the generator 61 for about 1 microsecond duration at a 1 percent duty cycle. This gating of the 4 to 8 GH signals is in synchronization with the bias pulse from source 77 to provide simultaneously at the amplifier 10 the 4 to 8 Gh signal and electric field bias.

The gated microwave signals were metered at power meter 75. The output from the amplifier 10 remained constant even though the input signal level changed. This indicated that triggered oscillations were taking place in the amplifier 10. The input power was metered by power meter 71 and oscilloscope 67. The amplifier, when operated at room temperature, was unstable and produced triggered oscillations, even when the device package 11 itself was biased by the bias course 77 with the bias pulse amplitude at 40 volts (over six times threshold).

The oven temperature was raised to about C. (Centigrade) and hence the ambient temperature of the bulk-type device was raised to about 75 C. This was measured by the thermocouple on heat sink 19. Without any output from the frequency generator 61 and with the amplifier 10 biased with a 40 volt pulse from source 77, no output power was noted at the amplifier 10. When the gated 4 to 8 GH signals were then applied from the sweep generator 61 in synchronization with the 40 volt bias. pulse, a linear amplification of in oll 50 milliwatts 88 milliwatts milliwatts 168 milliwatts 200 milliwatts 304 milliwatts 0.25 watts 0.43 watts 0.5 watts 0.88 watts 1.0 watt 1.7 watts 2.0 watts 3.5 watts 3.0 watts 4.8 watts 4.0 watts 5.6 watts Referring to FIG. 1, the capacitive tuning screw 71 is positioned relative to the device package 11 to change the capacitance slightly in order to provide the wideband uniform amplification over the 4 to 8 GH,

frequency range.

In the test facility described above, the losses were about 0.6 db. The signal gain of the amplifier was about (0.6 db 2.3 db) 2.9 db. The above values were again obtained when the amplitude of the biasing pulse was lowered to 30 volts and the oven temperature was raised to 100 C. The above measurement of the ambient temperature of the diode was made by a thermocouple placed on the heat sink 19.

In the above described arrangement, the real load impedance of the device was measured to be about 20 ohms. Stability may be theoretically maintained by avoiding the condition where the total circuit reactance is not zero at a frequency where the total circuit resistance is less than or equal to zero. Such a frequency would be referred to as a critical frequency and the amplifier would oscillate. If the device were anti-resonant (shunt tuned), stability would require that the total parallel circuit conductance be positive at any frequency. This condition would be met by requiring that the resonant positive load conductance be larger than the small signal negative conductance. This criteria for the state of stability of a negative resistance amplifier has been discussed by F. Sterzer in the Proceedings of the IEEE, Vol. 57, No. 10 on pages 1,781 to 1,783, October 1969, in an article entitled Stabilization of Snpercritical Transfer Electron Amplifiers.

It has been demonstrated therefore that a stable pulsed transferred electron amplifier can be provided using supercritically doped material to obtain high power pulsed amplification when sufficient loading, sufficient operating bias and thermally induced stability are provided.

It is believed that as the duty cycle of the bias increases above that of 1 percent, the ambient temperature of the device may not have to be raised as high as that described above by the applicant to achieve stable amplification. Also, as the ambient temperature was increased, the amplitude requirement of bias pulse was lowered. This indicates that as the ambient temperature is increased, the amplitude of external bias required may be made lower. Also, as the duty cycle of the bias pulse increases, the above indicated that the amplitude of the external bias may be made lower. In addition to loading therefore, temperature plays an important part in stability. The temperature of the device can be changed by changing the bias voltage level or its duty cycle and by increasing the ambient temperature of the device.

What is claimed is:

l. A transferred electron amplifier for amplifying microwave signals at or near the transit time frequency, comprising: a bulk-type transferred electron semiconductor device having first and second terminals, said first and second terminal being on opposite ends of said device, said device having an active region wherein at room temperature the product of the doping density times the length of the active region between said terminals is greater than 5 X cm' means for applying a pulsed electric field bias across said device with a value at least five times that of threshold where a transfer of electrons from a high to low mobility sub-band in said active region occurs,

impedance means for loading said device, and

means for raising the ambient temperature of the device to at least about C. to stabilize said amplifier and produce in response to said microwave signals linearly amplified microwave signals.

2. A method of stabilizing a transferred electron amplifier for amplifying microwave signals at or near the transit-time frequency where the amplifier comprises a bulk-type semiconductor device having first and second terminals with said temiinals being at opposite ends of said device, said device having an active region wherein at room temperature the product of the doping density times the length of the active region between said terminals is greater than 5 X lO cm' comprising the following steps of stabilizing said amplifier for providing linear amplification,

applying a microwave frequency signal across said device,

supplying a pulsed electric field bias across the terminals of said device of a magnitude at least five times a threshold value where transfer of electrons form a high to low mobility sub-band in said device occurs, and

loading said device by a certain amount and maintaining said device at a temperature of at least 75 C. at which oscillations of said device are suppressed.

UNITED STATES PATENT oTTTcE QERHNQATE or QCRRECHCN Patent No. 3,686,578 Dated August 22, 1972 Inventor(s) Chainulu L. Upadhyayula. and Barry S, Perlman It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 49, correct "resent" to read -present- Column 3, line 16, correct "Capacitor 20" to read --Capacitor 29---| Column 4, line 56, before "American" insert Column 4, line 60, correct "cm' to read --cm Column 6, line 1, correct "course" to read --source- Column 8, line 26, correct "form" to read -from--,

Signed and sealed this 30th day of January 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents FORM PO-IOSO (10-69) USCOMM-DC 60376-P69 3530 6'72 a u s. GOVERNMENY PRINYING OFFICE I969 o-3ss-a14 

1. A transferred electron amplifier for amplifying microwave signals at or near the transit time frequency, comprising: a bulk-type transferred electron semiconductor device having first and second terminals, said first and second terminal being on opposite ends of said device, said device having an active region wherein at room temperature the product of the doping density times the length of the active region between said terminals is greater than 5 X 1011cm 2, means for applying a pulsed electric field bias across said device with a value at least five times that of threshold where a transfer of electrons from a high to low mobility sub-band in said active region occurs, impedance means for loading said device, and means for raising the ambient temperature of the device to at least about 75* C. to stabilize saId amplifier and produce in response to said microwave signals linearly amplified microwave signals.
 2. A method of stabilizing a transferred electron amplifier for amplifying microwave signals at or near the transit-time frequency where the amplifier comprises a bulk-type semiconductor device having first and second terminals with said terminals being at opposite ends of said device, said device having an active region wherein at room temperature the product of the doping density times the length of the active region between said terminals is greater than 5 X 1011cm 2, comprising the following steps of stabilizing said amplifier for providing linear amplification, applying a microwave frequency signal across said device, supplying a pulsed electric field bias across the terminals of said device of a magnitude at least five times a threshold value where transfer of electrons form a high to low mobility sub-band in said device occurs, and loading said device by a certain amount and maintaining said device at a temperature of at least 75* C. at which oscillations of said device are suppressed. 